Codoped Direct-Gap Semiconductor Scintillators

ABSTRACT

Fast, bright inorganic scintillators at room temperature are based on radiative electron-hole recombination in direct-gap semiconductors, e.g. CdS and ZnO. The direct-gap semiconductor is codoped with two different impurity atoms to convert the semiconductor to a fast, high luminosity scintillator. The codopant scheme is based on dopant band to dopant trap recombination. One dopant provides a significant concentration of carriers of one type (electrons or holes) and the other dopant traps carriers of the other type. Examples include CdS:In,Te; CdS:In,Ag; CdS:In,Na; ZnO:Ga,P; ZnO:Ga,N; ZnO:Ga,S; and GaN:Ge,Mg.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority of Provisional Application Ser. No.60/411,491 filed Sep. 16, 2002, which is herein incorporated byreference.

STATEMENT OF GOVERNMENTAL SUPPORT

The United States Government has rights in this invention pursuant toContract No. DE-AC03-76SF00098 between the United States Department ofEnergy and the University of California.

BACKGROUND OF THE INVENTION

The invention relates to scintillators, and more particularly to fast,bright inorganic scintillators based on direct-gap semiconductors.

Inorganic scintillators have been used as radiation detectors forx-rays, gamma rays, and neutrons, for over a century. The history ofinorganic scintillators is described in S. E. Derenzo et al., “The Questfor the Ideal Inorganic Scintillator,” Nuclear Instruments and Methodsin Physics Research A 505 (2003) 111-117, which is herein incorporatedby reference.

Table I lists the best available prior art scintillators and comparesthem to the ideal. TABLE I Properties BGO Nd:T1 BaF₂ LSO LaBr₃:Ce Idealphotoelectric fraction* 0.43 0.18 0.19 0.34 0.14 >0.43 Density 7.1 3.74.9 7.4 5.3 >7 Photon/MeV 8,200 40,000 1,800 20,000 61,000 >100,000Energy resolution** 13% 6% 10% 11% 3% <3% Decay time (ns) 300 230 <1 4035 <1 Photons/ns/MeV*** 26 180 2000 500 1750 >100,000*σ_(photo/()σ_(photo +) σ_(Compton)) at 511 keV**fwhm at 511 keV***Photons/ns/MeV* = (photons/MeV)/(decay time)

FIG. 1 shows the physical processes active in the various common priorart scintillators. These include self-trapped excitonic scintillatorssuch as BaF₂; scintillators with activator ions such as NaI(Tl),CsI(Tl), LSO (Lu₂SiO₅:Ce), and LaBr₃:Ce: self-activated scintillatorssuch as BGO (Bi₄Ge₃O₁₂); and core-valence scintillators such as BaF₂.Certain phosphors have ultra-fast decay times, e.g. 0.7 ns for ZnO:Gaand 0.2 ns for CdS:In. Unfortunately, no known inorganic scintillator isboth bright and fast.

Direct-gap semiconductors are fast, luminous scintillators when cooledto cryogenic temperatures, but have greatly reduced luminosity at roomtemperature due to carrier trapping on non-radiative centers (crystaldefects and impurities). The need for cryogenic temperatures limitstheir use. A scintillator that is bright and fast at room temperature isdesired.

Positron Emission Tomography (PET) is able to measure the concentrationof labeled compounds in the human body as a function of time and is anefficient and accurate method for measuring regional biochemical andphysiological functions. It has been useful for the study of heartdisease, brain disease, and cancer. However, PET has been limited by thetiming and energy resolution of available scintillators. A scintillatorwith properties close to the fundamental limits of 1 ns decay time, 200ps fwhm timing resolution, and 3% fwhm energy resolution wouldsubstantially improve PET performance. Thus a new inorganic scintillatorfor detecting 511 keV photons with exceptional detection efficiency(like BGO), response time (1 ns), and energy resolution (3% fwhm) wouldallow septaless time-of-flight PET to be achieved. Fast brightscintillators could also enable an efficient time-of-flight gammadetector useful in other fields, such as nuclear physics, high energyphysics, and astrophysics.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the invention to provide an improvedfast bright inorganic scintillator that is operative at roomtemperature.

The invention is a new class of inorganic scintillators based ondirect-gap semiconductors. The direct-gap semiconductor is codoped withtwo different impurity atoms to convert the semiconductor to a fast,high luminosity scintillator. The codopant scheme is based on dopantband to dopant trap recombination. One dopant provides a significantconcentration of carriers of one type (electrons or holes) and the otherdopant traps carriers of the other type. Thus one dopant produces lotsof one carrier (e.g. electrons) and the other dopant traps the othercarrier (e.g. holes). The direct-gap semiconductor codoped with the twotypes of impurity atoms converts ionization energy into fastscintillation light with high efficiency at room temperature.

The scintillator material of the invention is generally described by theformula S:X,Y where S is a direct-gap semiconductor, and X and Y are thecodopants. The dopants are generally introduced at the 0.1 mole % to 0.2mole % level, but can more generally range from 0.01 mole % to 1 mole %.The dopant band to dopant trap recombination mechanism can beimplemented in four different embodiments by the selection ofappropriate dopants X, Y.

These embodiments are: 1. Donor band to acceptor trap recombination inwhich the donor dopant produces electrons in a donor band, e.g. becomesbulk n-type, and the acceptor dopant traps holes until they recombinewith the electrons in the donor band. 2. Acceptor band to donor traprecombination in which holes are produced in an acceptor band by theacceptor dopant, e.g. bulk p-type material, and recombine with electronstrapped on donor dopant atoms. 3. Donor band to isoelectronic(isovalent) hole trap recombination, similar to type 1 but the holes aretrapped on an isoelectronic impurity atom. 4. Acceptor band toisoelectronic electron trap recombination, similar to type 2 but theelectrons are trapped on an isoelectronic impurity atom. The first typeis generally preferred.

Specific scintillator materials of the invention include CdS doped withany of In, Ga, or Al and codoped with any of Te, Ag, Na or Li. CdS:In,Teis an example of type 3 and CdS:In,Ag; CdS:In,Na; and CdS:In,Li areexamples of type 1. ZnO doped with Ga and codoped with any of P, N, or Sis another specific scintillator material of the invention. ZnO:Ga,P andZnO:Ga,N are examples of type 1 and ZnO:Ga,S is an example of type 3.GaN codoped with Ge, Si, S or Se, and Mg are other examples of type 1.ZnTe doped with Mg forms type 2, 4 materials when codoped with asuitable trap.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic energy level diagram for the common scintillationmechanisms of prior art scintillators.

FIGS. 2-3 are schematic energy level diagrams for the scintillationmechanisms of the present invention.

FIGS. 4-6 respectively are the room temperature time spectra of CdS:In(prior art), CdS:Te (prior art), and CdS:In,Te of the present invention.

FIG. 7 is the room temperature wavelength spectrum of CdS:In, CdS:Te,and CdS:In,Te.

FIGS. 8-9 respectively are the room temperature time spectra ofCdS:In,Ag and CdS:In,Na of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention is an improved inorganic scintillator based on adirect-gap semiconductor into which two different impurity atoms ordopants are introduced. By selecting one foreign atom to provide acarrier band and the other foreign atom to provide for efficienttrapping of the other carrier, rapid radiative recombination occurs,providing a fast luminous scintillation mechanism.

The codoped direct-gap semiconductor scintillators work on the basis ofradiative electron-hole recombination. One dopant provides an abundantsupply of charge carriers of one type (electrons or holes). The otherdopant promptly and efficiently traps the other type of charge carrier(holes or electrons, respectively) produced by the ionizing radiation.The first dopant is a neutral donor or acceptor to provide an abundantsupply of either electrons or holes. The second dopant can be an ionizedacceptor atom (to trap holes), an ionized donor atom (to trapelectrons), or an isoelectronic trap (of holes or electrons), dependingon the first dopant.

For example, an electron donor (n-type) dopant provides a band ofelectrons near the bottom of the conduction band that rapidly combinewith holes produced by an ionization event to produce scintillationlight with a very short decay time. But with only a single electrondonor dopant, trapping of ionization holes on nonradiative centerslimits room temperature luminosity. However, the addition of the seconddopant to efficiently trap the holes produced in the ionization eventallows the holes to combine radiatively with the donor band electrons,producing scintillation with short decay time and high luminosity(brightness).

FIGS. 2-3 illustrate the scintillation mechanisms of the presentinvention.

In FIG. 2, a direct-gap semiconductor is codoped with (1) impurity atomsto provide a donor band of electrons, and (2) impurity atoms to trapionization holes, for rapid radiative recombination. The second impuritymay be an isoelectronic trap. High luminosity requires that the dopanthole traps are more efficient than the nonradiative recombinationcenters formed by crystal defects and impurities.

In FIG. 3, a direct-gap semiconductor is codoped with (1) impurity atomsto provide an acceptor band of holes, and (2) impurity atoms to trapionization electrons, for rapid radiative recombination. The secondimpurity may be an isoelectronic trap. Again, for high luminosity, thedopant electron traps must be more efficient than the nonradiativerecombination centers.

Thus the invention is implemented with a pair of dopants; one is a donoror acceptor of one charge carrier (i.e. either n-type or p-type doping),and the other is a trap for the other charge carrier. The mechanism ofdopant band to dopant trap recombination can be carried out by donorband to acceptor trap recombination, acceptor band to donor traprecombination, or dopant band to isoelectronic trap recombination,depending on the particular dopants chosen.

In donor band-acceptor trap recombination, the direct-gap semiconductoris doped with impurity donor atoms to become bulk n-type. This requiresthe donor atom to have an ionization energy low enough, e.g. about 30meV, to provide delocalized electrons in a donor band. The semiconductoris codoped with impurity acceptor ions that efficiently trap holesproduced by ionizing radiation until they recombine radiatively with theelectrons in the donor band. This requires the acceptor atom to have anionization level low enough, e.g. <about 200 meV, for efficient holetrapping but high enough, e.g. >about 50 meV, to avoid thermal trappingbefore radiative recombination occurs. The level of nonradiative centersshould be sufficiently low to avoid interference.

In acceptor band-donor trap recombination, the direct-gap semiconductoris doped with impurity acceptor atoms to become bulk p-type. Thisrequires the acceptor atom to have an ionization energy low enough, e.g.about 30 meV, to provide delocalized holes in an acceptor band. Thesemiconductor is codoped with impurity donor ions that efficiently trapelectrons produced by ionizing radiation until they recombineradiatively with the holes in the acceptor band. This requires the donoratom to have an ionization level low enough, e.g. <about 200 meV, forefficient electron trapping but high enough, e.g. >about 50 meV, toavoid thermal trapping before radiative recombination occurs. The levelof nonradiative centers should be sufficiently low to avoidinterference.

Dopant band to isoelectronic trap recombination, is similar to the abovebut the trap is an isoelectronic trap that localizes either the electronor hole on a single atom.

Specific requirements for the invention, in particular for an improvedscintillator for PET, include the following.

A. The semiconductor has a high atomic number and high density (similarto BGO) so that the 511 keV annihilation photons lose all their energyby photoelectric absorption in a small detector volume with highprobability. This is important because it provides a combination of finespatial resolution and high detection efficiency. Semiconductors haverelatively low melting points, and this contributes to low crystalproduction cost.

B. The semiconductor has a band-gap above about 2 eV so thatscintillation light can be detected by photomultiplier tubes.

C. The semiconductor has a direct gap, which means that the momentumspace vectors of an electron at the bottom of the conduction band and ahole at the top of the valence band are equal, and no transfer ofmomentum to the lattice is needed to generate an optical photon.Direct-gap semiconductors include ZnO, CdS, PbI2, HgI2, CuI, ZnTe, andGaN.

D. One impurity consists of donor ions with excess positive chargecompensated with a band of electrons near the bottom of the conductionband (FIG. 2) or acceptor ions with excess negative charge compensatedby a band of holes near the top of the valence band (FIG. 3).

E. The other impurity consists of ions that promptly and efficientlytrap ionization holes (FIG. 2) or electrons (FIG. 3) until they canradiatively recombine with the donor band electrons or acceptor bandholes respectively.

The scintillator material of the invention is generally described by theformula S:X,Y where S is a direct-gap semiconductor, and X and Y are thecodopants. There are four variations of the donor band to donor traprecombination mechanism, depending on the particular dopants X, Yselected: 1. Donor band to acceptor trap recombination in which thedonor dopant produces electrons in a donor band, e.g. becomes bulkn-type, and the acceptor dopant traps holes until they recombine withthe electrons in the donor band. 2. Acceptor band to donor traprecombination in which holes are produced in an acceptor band by theacceptor dopant, e.g. bulk p-type material, and recombine with electronstrapped on donor dopant atoms. 3. Donor band to isoelectronic(isovalent) hole trap recombination, similar to type 1 but the holes aretrapped on an isoelectronic impurity atom, i.e. the valence of thedopant is the same as the valence of the atoms it replaced. 4. Acceptorband to isoelectronic electron trap recombination, similar to type 2 butthe electrons are trapped on an isoelectronic impurity atom. The dopantsare generally introduced at the 0.1 mole % to 0.2 mole % level, but canmore generally range from 0.01 mole % to 1 mole %. The amount of eachdopant may be different.

CdS doped with any of In, Ga, or Al and codoped with any of Te, Ag, Naor Li is one particular direct-gap semiconductor scintillator materialof the invention. CdS:In,Te is an example of type 3 and CdS:In,Ag;CdS:In,Na; and CdS:In,Li are examples of type 1. ZnO doped with Ga andcodoped with any of P, N, or S is another particular direct-gapsemiconductor scintillator material of the invention. ZnO:Ga,P andZnO:Ga,N are examples of type 1 and ZnO:Ga,S is an example of type 3.GaN codoped with any of Ge, Si, S, and Se, and with Mg are otherexamples of type 1. ZnTe doped with Mg can be used to form type 2 and 4.The Mg dopant is an acceptor; a suitable donor electron trap orisolectronic electron trap is required as a codopant.

A particular example (of type 1) is ZnO:Ga,P where the direct gapsemiconductor ZnO is codoped with Ga and P. The Ga dopant produces anabundant supply of electrons while the P codopant traps holes. Holesproduced by ionizing radiation are promptly trapped on trivalentphosphorous ions and recombine with abundant electrons produced by thegallium.

Another example (of type 3) is CdS:In,Te where the direct gapsemiconductor is CdS and the codopants are In and Te. Indium is anelectron donor and tellurium is an isoelectronic hole trap. Indium makesthe material bulk n-type which provides a band of donor electrons nearthe bottom of the conduction band. When an electron or x-ray producesholes and electrons in the material, many of the holes are promptlytrapped in less than 0.05 ns on tellurium atoms and then recombine withelectrons from the donor band to produce scintillation light with adecay time of 3.5 ns.

The room temperature time spectra of CdS:In, CdS:Te, and CdS:In,Te areshown in FIGS. 4-6 respectively. CdS:In has a fast decay time, 0.2 ns,but has low luminosity at room temperature. CdS:Te has a slow decay timeof 3 μs. The codoped CdS:In,Te has a fast decay time of 3.5 ns andretains the emission wavelength of CdS:Te as shown in FIG. 7.

FIGS. 8-9 respectively are the room temperature time spectra ofCdS:In,Ag and CdS:In,Na of the present invention. The codoped CdS:In,Aghas a fast decay time of 1.6 ns compared with >5 μs for CdS:Ag, and thesame emission wavelength.

The codoped semiconductor scintillator materials of the invention can bemade by known methods. U.S. Pat. No. 3,534,211 to Lehmann describes amethod for producing n-type ZnO (singly) doped with Ga, In, or Al. Themethod can be adapted to make the codoped materials of the presentinvention by simply adding both dopants in place of the single dopant.For example, ZnO(Ga,P) has been prepared by a modified Lehmann processby incorporating 0.2 mole % of GaP into the ZnO+0.3 mole % Ga2O3 mix atthe beginning of the process.

An alternative process to reliably produce (singly doped) ZnO(Ga) withthirty times higher conversion efficiency than the Lehmann process iscarried out as follows. This process uses non-aqueous thermally induceddiffusion and avoids the more complicated aqueous chemistry of Lehmann.

1. Commercially available high purity (5N) ZnO powder is mixed with0.001 mole % to 1 mole % [or 0.1 to 0.2 mole %] high purity (5N) Ga2O3.

2. The Ga2O3 is incorporated into the ZnO by adding methanol andgrinding in an alumina mortar and pestle for typically 15 minutes untilthe mixture appears dry.

3. The mixture is then transferred to a glass container and placed in anoven at 120 C to dry completely for at least 10 minutes.

4. The mixture is heated in a furnace at 1100 C in a loosely cappedcontainer in vacuum (using a mechanical vacuum pump) for 10 to 15 hours.

5. The mixture is cooled down to room temperature, removed from thecontainer and ground dry with the mortar and pestle.

6. The mixture is heated in a furnace at 900 C in a loosely cappedcontainer in vacuum (using a mechanical vacuum pump) for 10 to 15 hours.

7. The mixture is cooled down to room temperature, removed from thecontainer and subjected to a light dry grinding with the mortar andpestle

8. The mixture is heated in a furnace at 800 C in a loosely cappedcontainer in flowing forming gas (3% hydrogen/97% Argon) for 30 minutes.

This process for making a material with a single dopant can again bereadily adapted to the present invention to make codoped materials bysimply adding both dopants in place of the single dopant in step 1.

Samples of CdS(In; Te, Ag or Na) were prepared from CdS powder (StremChemicals, 99.999% Puratrem), In powder 325 mesh (Alfa Aesar, 99.999%Puratronic), CdTe powder (Strem Chemicals, 99.999%), Ag2S powder, andNaCl powder. CdS(In) was prepared by incorporating 1% In into CdS by:(1) mixing the dry powders in a vortex mixer, (2) sealing the result ina quartz ampoule under vacuum with a residual pressure of 5×10−5 torr,(3) placing the ampoule in a tube furnace at 900 C for 10 hours, (4)cooling the ampoule at a rate of about 200 C/hour, (5) crushing theresulting material into a fine powder using a mortar and pestle, and (6)repeating steps (2) to (5). CdS(Te), CdS(Ag), and CdS(Na) were preparedby incorporating 0.01% CdTe, 0.05% Ag2S, or 0.05% NaCl, respectively,into CdS using steps (1) to (5). CdS(In,Te), CdS(In,Ag), CdS(In,Na) wereprepared by combining equal amounts of CdS(Te), CdS(Ag), or CdS(Na),respectively, and CdS(In) and performing steps (1) to (5). [All dopantpercentages are mole fractions.]

The invention is useful for the detection of ionizing radiation wherefast response and high data rates are important. Applications includemedical imaging, nuclear physics, nondestructive evaluation, treatyverification and safeguards, environmental monitoring, and geologicalexploration. In particular, the new inorganic scintillator will allowseptaless time-of-flight PET to be achieved. This will be a majorimprovement, providing much finer resolution, higher maximum eventrates, and clearer images.

Changes and modifications in the specifically described embodiments canbe carried out without departing from the scope of the invention whichis intended to be limited only by the scope of the appended claims.

1.-7. (canceled)
 8. An inorganic scintillator comprising: a direct-gapsemiconductor and a pair of codopants in the semiconductor to providedopant band to dopant trap radiative recombination, wherein one codopantis an acceptor dopant which produces holes in an acceptor band and theother dopant is a donor dopant trap which traps electrons until theyrecombine with holes in the acceptor band. 9.-11. (canceled)
 12. Aninorganic scintillator comprising: a direct-gap semiconductor; and apair of codopants in the semiconductor to provide dopant band to dopanttrap radiative recombination, wherein one codopant is an acceptor dopantwhich produces holes in an acceptor band and the other codopant is anisoelectronic dopant trap which traps electrons until they recombinewith holes in the acceptor band. 13.-16. (canceled)
 17. An inorganicscintillator comprising: a direct-gap semiconductor; and a pair ofcodopants in the semiconductor to provide dopant band to dopant trapradiative recombination, wherein the direct-gap semiconductor and pairof codopants comprises CdS:In,Te; Cd:In,Ag; or Cd:In,Na. 18.-19.(canceled)
 20. The scintillator of claim 8, wherein each codopant ispresent at about 0.01 mole % to about 1 mole %.
 21. The scintillator ofclaim 8, wherein each codopant is present at about 0.1 mole % to about0.2 mole %.
 22. The scintillator of claim 8, wherein the direct-gapsemiconductor is PbI₂, HgI₂, CuI, or ZnTe.
 23. The scintillator of claim8, wherein the direct-gap semiconductor is ZnTe.
 24. The scintillator ofclaim 12, wherein each codopant is present at about 0.01 mole % to about1 mole %.
 25. The scintillator of claim 12, wherein each codopant ispresent at about 0.1 mole % to about 0.2 mole %.
 26. The scintillator ofclaim 12, wherein the direct-gap semiconductor is PbI₂, HgI₂, CuI, orZnTe.
 27. The scintillator of claim 12, wherein the direct-gapsemiconductor is ZnTe.